In recent years, the wireless communication market has expanded greatly. Wireless devices, such as remote control engine start systems, remote keyless entry (“RKE”) systems, and automatic tolling systems are now considered “classical” devices for short range vehicle wireless communication. Such control and security devices commonly operate in the 315 MHz frequency in the United States, Canada, and Japan. In these systems, the antenna is a key element in determining system size and performance. Examples of external and internal antennas that are in current production are known. As a rule, internal antennas are printed on dielectric boards together with electronic components of RKE systems, for example. The integration of radio frequency (“RF”) and digital electronic components with receiving antennas reduces the number of wires and connectors, thus reducing system costs. Nevertheless, such designs have a significant disadvantage, namely parasitic emissions from electronic components (oscillators) located on the circuit board that can markedly reduce the communication range.
An external dipole antenna does not have such a disadvantage because it is isolated from the elements of the control electronics. Unfortunately, such antennas with lengths of about 30 cm are large and inconvenient for interior vehicle applications. The “pigtail” coaxial antenna described in U.S. Pat. No. 6,937,197 avoids some of the problems seen in external dipoles, and thus may be more convenient for automotive interior applications. The pigtail is made by simply stripping off the outer conductor of the coax to extend the inner conductor to a length equal to approximately a quarter-wavelength; the cable becomes a part of the antenna. One problem associated with pigtail antennas is that in automotive applications pigtail antennas are positioned very close to the car body as a part of a cable harness. Because of the metal shadows from the car body, the pigtail has very small gain; the small gain in turn causes reduced communication range. Therefore, in applications where communication range is a critical factor, pigtail antennas are not acceptable for automotive antenna applications.
Referring to FIG. 1, a conventional asymmetrical meander antenna 50 is shown, which tends to have a significant current flow in the outer conductor of the RF cable 54 that connects the asymmetrical meander antenna 50 with a control module, such as remote keyless entry (RKE”) module 58. Asymmetrical meander antenna 50 includes asymmetrical trace lines 52 that are printed on a printed circuit board (PCB”). Essentially, the RF cable 54 becomes part of the asymmetrical meander antenna 50 and provides for extended signal range. A drawback of such asymmetry is that the cable location influences the communication range of the RKE system. Modern vehicles have many different electronic devices, including heaters, air conditioning modules with automatic temperature control, audio amplifier systems, heated seat modules, power control modules, and sunroof modules, for example. Parasitic emissions from these electronic devices near the routing path of the external antenna's RF cable can reduce the communication range of the asymmetric RKE system. In fact, electromagnetic compatibility (“EMC”) measurements show that such interference can exceed the noise floor level of the RKE system by more than 20 dB.
In one example, a nominal communication range for asymmetrical RKE systems is approximately 100 m in the absence of parasitic emissions. Experimental measurements show that the noise received by the RF cable can exceed the noise floor of the RKE by 20 dB. Such noise level reduces the communication range of the RKE systems to 20 m or less. Generally, the effect of parasitic components on a cable can be minimized by using a special passive electronic device, such as a balun, for balancing impedances, between the antenna and RF circuit. Nevertheless, such a printed-on-circuit-board balun has a linear size equal to a quarter of the wavelength, and therefore is generally too large for automotive applications operating at 315 MHz. Therefore, automotive designers are forced to use antennas without a balun.